Blastocystis hominis

Blastocystis hominis is an anaerobic intestinal protozoan
originally allied with the amoebae and more recently with the
alveolates (Apicomplexa, dinoflagellates, and Ciliophora). Numerous
morphologically similar forms exist
in different animals and the number of true species is not yet
known. At least three distinct morphological
types can be found in feces, including the common vacuolar stage
(above) which divides, an amoeboid stage which may be invasive and is
capable of budding,
and a granular or cystic form which is thought to give rise to numerous
new organisms when conditions become unfavorable. Heavy infections are
linked to diarrhea and intestinal discomfort, but whether the parasite is
a causative agent of intestinal distress or responds favorably to an
opportunistic situation is still unclear. Certainly the detection of high
numbers in fecal smears is an indicator that something is amiss. The
parasite seems to be susceptible to numerous drugs, including
trimethoprim/sulfamethoxazole, metronidazole, and
pentamidine.